: The current renewal will determine the cellular mechanisms underlying agonist-induced sensitization and the biochemical, structural, and physiological consequences of pharmacologic overstimulation of cardiac fetal beta-receptors by terbutaline. They will then go on to evaluate what cellular and molecular events occur within the postnatal period to replace fetal agonist-induced sensitization with the mature mechanism, agonist-induced desensitization. The aims are: 1. To determine what cellular mechanisms govern the ontogeny of agonist-induced beta-receptor downregulation and uncoupling from response elements in the developing heart; 2. To determine the mechanisms underlying fetal agonist induced sensitization of cardiac adenylyl cyclase; to determine the structural and functional consequences of fetal overstimulation with terbutaline during the period in which desensitization cannot be evoked.